There are many ways to measure body weights. For some people, it is not convenient to utilize normal platform scales to measure their body weights. Especially for the patients who cannot move themselves, measuring their body weights precisely and efficiently becomes a challenge of medical engineers. For some pressure mattresses, the pressure inside the mattresses can be adjusted according to the users' body weights. Therefore, measuring a user's (or a patient's) body weight is an important task. There are some methods utilizing an additional weight-measuring device arranged under a mattress to measure a patient's body weight. However, an additional device means additional cost, and the maintenance of the additional device is another burdensome extra-work. Thus, convenient and efficient apparatus and method for measuring a user's body weight is definitely needed.